Conventional methods for production of condensed silica aerogels use silica gels which are synthesized in a single step hydrolysis of a silica alkoxide, such as tetramethoxysilane (TMOS) with water in the presence of basic catalysts. ##STR1##
The condensation reaction step immediately follows hydrolysis to yield an alcogel, so the overall reaction rate is largely determined by the hydrolysis rate. The spheroidal macroparticles which grow from the condensed silica alcogel are surrounded by alcohol solvent which is typically removed by supercritical extraction, with high temperatures and pressures, in an autoclave.
A more recently described method for production of condensed silica is a two-step method which enables production of silica aerogels of very low density. A related U.S. application Ser. No. 07/571,061, describes this two-step method for making aerogels, especially those of very low density. In this process, a sub-stoichiometric amount of water is used in the hydrolysis reaction with an acid catalyst and solvent alcohol so that the metal alkoxide, in the preferred mode TMOS, is only partially hydrolyzed. The solvent alcohol and reaction-generated alcohol are removed by distillation, leaving a partially condensed silica intermediate. The partially condensed silica intermediate was stablized against further hydrolysis by dilution with a non-alcohol solvent. Silica aerogels having a final density as low as 0.003 g/cm.sup.3 could be made from these gels by further dilution of the partially condensed silica intermediate with large quantities of non-alcoholic solvent and by use of base catalyst for the condensation step. Further modifications enabled us to successfully produce transparent silica aerogels at any density less than 0.02 g/cm.sup.3, including ultra-low density aerogels which have a density in the range of about 0.003 g/cm.sup.3.
Casting silica aerogel slabs with specific sizes and shapes, especially aerogels with ultra-low densities, has been problematic in the past because of 1) the tendency of the gel to adhere to the mold surface, and 2) the tendency of the gel to shrink and warp during the drying process. Additionally, irregular shapes occur in the castings due to the meniscus which forms on the free surface of the gel in the mold. Commonly used molding techniques are not satisfactory. When glass molds are used, the mold must be coated with a releasing compound to prevent sticking of the gel to the casting. The same is true for stainless steel molds. Teflon molds do not stick to the aerogel casting, but teflon molds deform at the high temperatures required for extraction of solvent from the gel. Uncoated aluminum metal molds are dissolved by the solvent.